Absentee ballots are becoming increasingly popular, as tens of thousands of them were sent to Central Florida voters last week for the upcoming primary race.

While Republicans still run strong in this arena, Democrats are gaining ground. In Orange County, 70,503 absentee ballots were sent to Democrats last week, compared with 51,418 headed to Republican households. Another 31,394 went to non-affiliated or independent voters.

A variety of factors, including organizing and registration efforts, have helped drive up the absentee-request advantage Democrats see in Orange. In Osceola, Democrats also have a lead in mailed-out absentees. But in Seminole and Lake counties, Republicans still have the decisive edge in getting mail-in ballots to their faithful.

"In the last election cycles, the Democrats have caught up," said Alan Byrd, who does consulting for Central Florida candidates in both parties. "It's the nature of the business. The absentee ballot is an absolute critical part of the ground game."

In Osceola County, 26,362 absentees were sent, with 49 percent of them delivered to Democrats and 28 percent to Republicans. That mirrors registration numbers there, where Democrats outnumber Republicans 43 percent to 25 percent, records show.

In Lake County, registered Republicans have climbed by more than 3,000 in the last four years, but the number of non-affiliated voters has increased by more than 7,000 people. Democratic registration has remained flat.

Likewise, Republicans in Lake far outpace Democrats by healthy margins in absentee voting. Almost 53 percent of the mail-in ballots requested by Lake voters for the upcoming primary went to Republicans, even though they make up 44 percent of the electorate.

In Seminole County, a GOP stronghold, more than 43,000 absentees were mailed to voters last week, with almost half of them going to Republicans. Democrats received only 32 percent of the mail-in ballots.

The growing popularity of mail-in voting is clearly evident there. In the 2006 primary, Seminole saw nearly 8 percent of its votes come in through the mailbox. In the 2012 primary elections, that number had jumped to more than 32 percent, according to figures from Election Supervisor Mike Ertel.

The number of people who showed up to vote on Election Day for the Seminole 2006 primary races was almost 80 percent. By the 2012 primary, that total had dropped to about 52 percent.

For Central Florida candidates, the popularity of absentee and early voting means a longer run to chase after votes.

"A campaign now is basically a 45- or 60-day marathon, rather than a two-week sprint," said Lonnie Thompson, of True Results Communications and Consulting. "You can't wait until a Election Day any more."

University of Wisconsin-Madison political science professor Barry Burden studies voting behaviors and in recent years found that states are consistently expanding early voting options.

"Once states open the doors to absentees and early voting, it grows," Burden said.

Burden was part of research team that concluded that expanded early-voting options were not necessarily increasing turnout, and may be reducing it overall.

One reason: With more voters casting ballots before Election Day, parties scale back their last-minute turnout efforts, and there's less civic, water-cooler or media buzz around that one day of voting in person.

"With significant early voting, Election Day can become a kind of afterthought, simply the last day of a drawn-out slog," Burden co-wrote in a 2010 op-ed piece in the New York Times. He and his colleague pointed to same-day registration as one key way to drive up turnout, something Florida does not allow.

Orange County Democratic Party chief Carlos Smith said that state internal tracking numbers show that Orange ranks among the top counties along with Miami-Dade and Pinellas, in terms of putting absentee ballots into voters' hands.

"That's huge and we're continuing our work in those areas to pull even further ahead of Republicans than we already are," Smith said.

Orange Election Supervisor Bill Cowles said local absentee request numbers have partly been boosted by competitive city races this year in Orlando and Apopka, which both saw strong mail-in drives by their local candidates.

Orange County Republican Party Chairman Lew Oliver said that despite strides made by Democrats in the absentee chase, it's not showing up in notable turnout advantages at the end of elections. As many as 90 percent of new absentee voters may just "cannibalize" turnout on Election Day, Oliver said.